First, some background. This is the classic proof-text for the concept of "venial" vs. "mortal" sins in the Roman Catholic Church. A "venial" sin is said to be minor, such that it will not cause you to go to hell if unrepented of and unforgiven (i.e. not confessed to a Roman Catholic priest during Roman Catholic Mass), although you will have to do time in Purgatory for it.
The Roman Catholic "mortal" sin, if not confessed, repented of, and forgiven (through Roman Catholic Mass) will cause you to go to hell.
Now, the Protestants recognized salvation by faith alone. However, still viewing sins as either grievous or trivial, they felt that there were sins committed that were bad enough to send a person to hell if they did not repent of them, and that we shouldn't even pray concerning that.
Now, we know that all sins are forgiven. Jesus paid for them all, the big sins and the little sins. The only sin now which sends one to hell is unbelief, the fact that you do not believe and do not accept that your sins are forgiven, therefore rejecting the Word of God.
So, still keeping with the traditional English translation of 1 John 5:16-17, we would have to conclude that the "sin that leads to death" would be unbelief; you cannot force a person to believe against their will, and you cannot "pray" to force their free will. People will suffer eternal death for refusing to believe, refusing to accept God's forgiveness for sins.
Whether they commit sins is a heart issue, tied into faith vs. unbelief. Faith in the forgiveness of sins brings regeneration by the power of the Holy Spirit and His indwelling, causing one to not desire to sin, but to please God. In the reverse, one who sins habitually and without regret demonstrates evidence of their unbelief (to all creation) by doing so.
Now, if you are content to stick with the above, you should do fine. End of discussion.
But upon a closer examination of 1 John 5:16-17 in the original language, I found a different story, both contextually and in the original words and grammar (although it does not contradict what I just explained above).
First of all, if your translation says "sin leading to death," the word "leading" is not in the original text. Neither is the word "God," as in "God will give him life." Also, the word "pray" is not in the text. These added embellishments in the English translations should be enough to raise a bit of a red flag, that we ought to look at it more closely.
Let's start with the context, backing up to verses 11 and 12:
And this is the testimony, that God gives to us life eternal, and this life is in his Son. The one having the son is having the life; the one not having the Son of God is not having the life.This is simple. Continuing in verse 13,
I write these to you, the ones believing into the name of the Son of God, that you may be having been aware that you are having life eternal and that you may be believing into the name of the Son of God.So, clearly, this is all about either believing or not believing, having the Son or not having the Son. It is very black and white.
Next, two key words to recognize are αιτησει ("aitesei"), in the first part of verse 16, which means "request," with an implied expectation of the request being granted, and ερωτηση ("erotese"), at the end of verse 16, which simply means "ask," like when you ask a question, but unfortunately this second word is often translated, "pray," loading it with the religious implication that you are praying to God.
The first word, "request," is used four times in the two verses before, verse 14-15:
"And this is the boldness that we have toward him, that if ever we should be requesting according to his will, he is hearing us. And if ever we have been aware that he is hearing us, whatever we may be requesting, we have been aware that we are having the request-effects that we have requested of him." (1 John 5:14-15)Again, very black and white. We request and we get what we request. This is the preface to the two verses in question.
Now, since the original text has no punctuation, I'll take my own liberties with the punctuation in the next two verses, and I think you'll see something completely different:
"If ever anyone may be perceiving his brother sinning a sin, not toward death shall he request, and he shall give life to him. To the ones sinning not toward death, sin is toward death. I am saying that he should not be asking about that. Every/all un-just-ness is sin, and is sin not toward death." (1 John 5:16-17)Again note: Neither "leading" nor "God" nor "pray" is in the text above. Check it in an interlinear and you will see.
This is still a little awkward, especially since I am making sure to be faithful to the Koine Greek grammar (at the expense of flowing nicely in English), and that is the problem with the original text in the first place, but you should start to see a different meaning popping out. First of all, "every/all un-just-ness is sin, and is sin not toward death." So there you have that, categorically, any unjust action is not a "sin toward death." This contradicts any Roman Catholic concept of "mortal sin," or any "Protestant" variation of it, for that matter.
In the context, the passage of scripture is talking about either having "life eternal" in the Son or not, depending on whether they believe. In particular, the person "sinning" is not "sinning toward death," but to him it is toward death. So that is a problem, that such a person thinks it is. But you won't fix it by asking about that. This brings us to the ultimate irony of this passage, which is that it tells us that we shouldn't be asking about the "sinning toward death" issue! The question should never have come up in anyone's mind! Why? Because, as we already know, Jesus paid for all sins. No one has a "sin problem" with God if all sins are paid for. They may have an "unbelief" problem with God, and that "unbelief" will most surely land them in hell, sinning sins all the way in the meanwhile, but a person is either "in Christ" or he is not. Christians, i.e. believers, do not go to hell. Unbelievers go to hell.
Now, let's put this all together, in context, which is about "eternal life" in the Son, not "eternal life" by abstaining from sin, or eternal death by committing a sin. Follow along in your favorite interlinear, if you have access to one:
"And this is the testimony, that God gives us eternal life, and this life is in his Son. The one having the Son is having the life; he who is not having the Son is not having the life. These I write to you, the ones believing into the name of the son of God, that you may be having been aware that you are having eternal life. And this is the boldness that we are having toward him, that if ever we should be requesting anything according to his will, he is hearing us. And if ever we have been aware that he is hearing us, whatever we may be requesting, we have been aware that we have the request-effects that we have requested of him. If ever anyone may be perceiving his brother sinning a sin, not toward death shall he request, and he shall give life to him. To the ones sinning not toward death, sin is toward death. I am saying that he should not be asking about that. Every/all un-just-ness is sin, and is sin not toward death. We have perceived that everyone having been begotten out of God is not sinning, but the one being begotten out of God is keeping himself and the wicked one is not touching of him." (1 John 5:11-18)I still translated it literally enough that it is somewhat awkward, but it should make sense now. This further supports what many of us have been saying, that we should speak life, not death to people. We are ministers of reconciliation (2 Cor 5:18), not judgment.
I grant this work to the public domain.